1. Field of the Invention
The present invention relates to an actuator, and more particularly to an actuator suitably used in an optical devices such as an optical scanner, an optical attenuator, and an optical switch.
2. Description of the Related Art
For example, polygon mirrors (rotary polyhedrons) are used as actuators for devices such as laser printers. In order to achieve high-speed printing with high resolution and high quality, it is necessary to rotate a polygon mirror at higher speeds. Currently, an air bearing is used to rotate a polygon mirror at high speeds with stability. However, it is difficult to further increase a rotational speed of a polygon mirror. Additionally, a large-sized motor is required to rotate a polygon mirror at high speeds. Such a large-sized motor is disadvantageous to miniaturization of the device. Further, use of such a polygon mirror needs a complicated structure and thus increases cost for the device.
There has been proposed an actuator having a relatively simple structure as shown in FIG. 1. The actuator shown in FIG. 1 employs a torsional vibrator having a single degree of freedom, in which flat electrode plates are arranged in parallel to each other. See K. E. Petersen, “Silicon Torsional Scanning Mirror,” IBMJ. Res. Develop., vol. 24 (1980), p. 631. Further, there has also been proposed an electrostatic drive type vibrator having a single degree of freedom in which the aforementioned torsional vibrator is modified so as to have a cantilever structure. See Kawamura et al., “Research in Micromechanics Using Si,” Proceedings of the Japan Society for Precision Engineering Autumn Conference (1986), p. 753.
The electrostatic drive type torsional vibrator with a single degree of freedom, as shown in FIG. 1, includes a glass substrate 1000, a movable electrode plate 300 made of monocrystal silicon, and spacers 200 disposed between the glass substrate 1000 and the movable electrode plate 300. The movable electrode plate 300 has fixing end portions 300a fixed to opposite ends of the glass substrate 1000 with the spacers 200 being interposed between the fixing end portions 300a of the movable electrode plate 300 and the glass substrate 1000. The movable electrode plate 300 has a movable electrode portion 300c and narrow torsion bars 300b for supporting the movable electrode portion 300c between the fixing end portions 300a. The torsional vibrator also includes a fixed electrode 400 disposed on the glass substrate 1000 so as to face the movable electrode portion 300c with a predetermined electrode interval. Specifically, the fixed electrode 400 is arranged in parallel to the movable electrode portion 300c. The torsional vibrator has a switch 600 and a power source 500 connected between the movable electrode plate 300 and the fixed electrode 400.
In the torsional vibrator thus constructed, when a voltage is applied between the movable electrode portion 300c and the fixed electrode 400, the movable electrode portion 300c is pivoted about the torsion bars 300b due to an electrostatic force. Since electrostatic attraction is inversely proportional to a square of an electrode interval, it is desirable that this type of electrostatic actuator has a small electrode interval between the movable electrode portion 300c and the fixed electrode 400. In the aforementioned structure having a single degree of freedom, however, the movable electrode portion 300c serves not only as an electrode but also as a movable portion. Accordingly, if an electrode interval is reduced, a movable range (rotational angle) of the movable electrode portion 300c is limited. On the other hand, a large electrode interval is required to maintain a large movable range. Thus, it is difficult to achieve both of low-voltage driving and a wide movable range (large deflection angle).